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Laser makes ultra-light mirror out of tiny beads

By Jacob Aron

Shooting a laser at polystyrene beads, scientists have made a mirror that is held together by light. The creation could be a step towards putting ultra-light mirrors in space that would be big enough to see continents and forests on planets orbiting far-off stars.

Current space telescopes have limited vision because is it costly and complicated to send large, heavy mirrors into orbit. The mirror on NASA’s premiere planet hunter, the Kepler space telescope, is just 1.4 metres across and cannot see planets directly. Instead Kepler spots the tiny changes in brightness when a world crosses in front of its host star.

When NASA’s James Webb Space Telescope launches in a few years, it will carry the largest mirror yet into space&colon; a 6.5-metre behemoth made of 18 interlocking segments. To fit into the launch vehicle, the mirror itself will have to be folded up and then unfolded in space.

Jean-Marc Fournier of the Swiss Federal Institute of Technology in Lausanne, Switzerland, and his colleagues have revived an old idea for building much larger mirrors by exploiting the force produced when laser beams hit tiny particles. Previous work has used this force to make optical tweezers, which can trap and manipulate a few particles at a time.

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Self-healing mirror

In 1979, astronomer Antoine Labeyrie, now at the Collège de France in Paris, suggested that the force could also trap a collection of particles into a flat plane to form a mirror. In theory, shooting two lasers at a central point should cause their optical forces to interfere, creating a stable region where particles line up to make a two-dimensional surface.

Such a mirror would be exceptionally light, relatively inexpensive and even self-repairing, as any particles knocked out by micro-meteors, which are constantly zipping through space, would simply be replaced by others nearby.

With funding from NASA’s Institute for Advanced Concepts, Fournier’s team took a first step towards this goal. They used a single laser to trap 150 micrometre-sized polystyrene beads against a sheet of glass (pictured). Light would normally bounce off a single bead in all directions, but grouping them together produces a flat reflective surface that acts exactly like a mirror, says Fournier.

To prove the mirror worked, the team shot light through a transparent ruler, so that it bounced off the beads and onto a detector. The resulting picture was murky, but they were able to make out an image of the number 8 on the ruler, which wasn’t possible when the beads were removed from the glass.

Spying on exo-Earths

Fournier thinks the technology could be scaled up to make a 35-metre mirror that would weigh just 100 grams, although he admits there are a number of hurdles to overcome before this technology can be used in telescopes. At the moment the beads are in water, which helps cool them and keep them together, but this wouldn’t be possible in the vacuum of space.

“The water is cheating, we know that,” he says. “At least it helps us move a little bit towards another step.” Replacing the glass with a second laser will also be a challenge, as will finding a cost-effective power source for the lasers.

“Whether this technology could be ready for a James Webb successor is quite speculative and would depend on many engineering and mission details that are not yet known,” says Jonathan Arenberg of Northrop Grumman Aerospace Systems in Redondo Beach, California, who is the chief engineer for the telescope.

Labeyrie says he would like to see the team repeat the experiment in a vacuum and in microgravity, perhaps on the International Space Station. If the technology holds up, he envisions sending up an array of laser-trapped mirrors that would act collectively like a single large one.

“Ten or 100 kilometres may become feasible in this way, and this can provide direct images of exo-Earths, where continents and forested areas such as the Amazon Basin become directly visible,” he says.